C++20 不再需要依赖作用域

Question

我最近升级到 c++20 并注意到当我不将 typename 放在依赖的 cope 类型别名前面时编译器不会抛出错误，例如

using iterator = (no typename here) std::vector<int>::iterator

这现在是新的 c++20 标准的一部分还是它只是 gcc 的东西而不是所有的编译器都这样做？

Answer 1

这是一个 C++20 的东西：

In some contexts, only type names can validly appear. In these contexts, a dependent qualified name is assumed to name a type and no typename is required:

• A qualified name that is used as a declaration specifier in the (top-level) decl-specifier-seq of:
  • a simple declaration or function definition at namespace scope;
  • a class member declaration;
  • a parameter declaration in a class member declaration (including friend function declarations), outside of default arguments;
  • a parameter declaration of a declarator for a function or function template whose name is qualified, outside of default arguments;
  • a parameter declaration of a lambda expression outside of default arguments;
  • a parameter declaration of a requires-expression;
  • the type in the declaration of a non-type template parameter;
• A qualified name that appears in type-id, where the smallest enclosing type-id is:
  • the type in a new expression that does not parenthesize its type;
  • the type-id in an alias declaration;
  • a trailing return type,
  • a default argument of a type template parameter, or
  • the type-id of a static_cast, dynamic_cast, const_cast, or reinterpret_cast.

有关详细信息，请参阅 dependent_name#The_typename_disambiguator_for_dependent_names。